The known art for the selective extraction of desired materials from feed streams falls into three general categories. First among these is resin bead extraction typified by ion exchange resins which have been used selectively to extract species such as transition metal ions from aqueous solutions. Specific chelating resins with selectivity for one or more desirable metal ions are also known but are difficult to manufacture, consequently expensive, and suffer from severe bead attrition due to physical breaking up of the beads in flow systems. The difficulty of attaching desirable chelating groups to available macroporous resin beads limits the quantity and selectivity of the materials which can be made using this technique.
One relatively recent publication "Copper-Selective Ion-Exchange Resin with Improved Iron Rejection" R. R. Grinstead, Journal of Metals,Vol. 31, No. 3 (1979) 13-16, describes a then improved chelating resin for copper/iron ion solutions, highly selective for copper.
The present invention overcomes the disadvantages above by using affinity adsorbents such as chelating polymers, ground ion exchange resins and enzymes all of variable and wide ranges of molecular weight or particle size, thus providing easier synthesis as well as eliminating the bead attrition problem. Additionally, the advantage of continuous operation can be easily realized with the present invention without the problems associated with fluidized bed systems for continuous ion exchange.
A second process involves solvent extraction which is typically used to recover mineral values using selective extractants soluble in solvents, immiscible with the feed solvent. Large volumes of solvent and extractant inventory are required, however, and valuable extractant is commonly lost due to solubility in the feed stream and entrainment losses resulting from microfine solvent/extractant particles which do not coalesce in the solvent/feed separators.
More recently, patent and open literature references have suggested the use of membranes as separators to avoid the solvent/extractant losses due to entrainment.
U.S. Pat. No. 3,808,267 teaches a membrane process for recovery of C.sub.2 to C.sub.4 carboxylic acids from dilute aqueous solution with one side of a microporous membrane while the other side is contacted with a liquid organic solvent for the acid.
"Membrane-Based Solvent Extraction for Selective Removal and Recovery of Metals", B. M. Kim, Journal of Membrane Science, 21 (1984) 5-19, describes a process for stripping industrial metals from wastewater. The process employs two modules, one for extraction through which the aqueous waste water stream flows, routed through thousands of very thin hollow fibers. An organic solvent containing a liquid ion-exchange material flows between the fibers and collects metal ions migrating through the pores in the fibers. The organic solvent stream then flows through a stripping module wherein an acid, base or salt solution, as appropriate, removes the metal ions.
In both cases these references discuss the use of immiscible solvents as the means of separating the extractant from the feed stream since the extractants used are small enough to permeate through the membranes employed. Both solvent and extractant soluble losses do occur in these systems and result in organic pollution of the feed stream. Inasmuch as the membranes in the present invention can be selected and operated in a manner which will prevent loss of the polymeric adsorbent as will be detailed hereinbelow, such organic pollution is eliminated. It is believed that an improvement in permeation rate may also be realized due to the lack of a solvent/feed interfacial barrier in the subject process.
Membrane methods comprise the third category for the selective removal of materials from solutions. In most instances these methods have been pressure driven ultrafiltration or reverse osmosis processes as opposed to the concentration gradient driven process of the present invention. U.S. Pat. No. 4,163,714 describes the preparation of pressure driven affinity sorption membranes from a membrane filter which is composed of an insoluble matrix polymer or interpolymer complex. The membranes are employed by passing material-containing solutions therethrough where one or more of the materials forms a complex with the ligand on the membrane pore surface and is subsequently displaced in a concentrated state.
In any pressure driven process with a polymeric material or suspension, a process known as concentration polarization occurs which results in substantial loss of permeation due to fouling of the membrane surface. This phenomenon limits the concentration of polymer or suspended absorbent which can be efficiently used. Inasmuch as the invention described here does not require a significant transmembrane pressure or liquid flux across the membrane, this polarization or fouling process will not occur.
An additional means of selective extraction using membranes involves the use of a selective solvent extractant, immiscible with both the feed solution and the liquid product stream, which is imbibed into the pores of a porous membrane. These systems are prone to loss of selectivity due to excessive transmembrane pressure, forcing the organic solvent out of the pores and developing a leak across the membrane. Another common problem is the maintenance of the solvent in such a small volume relative to the flow streams on both sides of the membranes and also, the slight solubility of the extractant and solvent in the effluent streams requires periodic replacement of either the extractant fluid or the entire membrane itself. The subject invention does not involve a selective membrane or solvent, but instead a selective polymer and minimal losses thereof can easily be replaced in a continuous flow loop during normal operation as necessary.
Despite the widespread existence of different processes for the selective extraction of various materials, the art has not recognized heretofore a process wherein a permeable membrane is employed in conjunction with a particular affinity adsorbent that has a binding affinity for the material to be removed. As a result, the process of the present invention does not require chelating ion-exchange resins, which are difficult to manufacture; physical attrition of resin beads is avoided; and, there is no loss of polluting solvents or expensive extractants which characterizes solvent extraction and liquid filled selective membrane processes.